The present invention concerns a free-cutting steel. More specifically, the invention concerns a free-cutting steel, in which Pb-content is less than the detectable limit, and therefore, which can be said Pb-free, but still machinability, particularly, machinability in turning, is excellent and the surface roughness after turning is in small.
To date, screws and nipples to which high strength is not required are manufactured by machining a free-cutting steel chosen as the material. In general, free-cutting steels are low carbon steels with addition of a machinability-improving element such as S, Pb, Te and Ca. Of these machinability-improving elements Pb is notorious by its effect of improving machinability in turning without damaging strength of the steel.
However, in view of the fact that Pb is a substance giving unfavorable influence to the environment, it is demanded to make the free-cutting steel Pb-free. Thus, efforts for developing free-cutting steel containing substantially no Pb but exhibiting machinability equal to or even better than that of the conventional Pb-containing free-cutting steel has been made. An example of such a free-cutting steel is a low carbon sulfur-free-cutting steel containing no Pb but S in an amount of, by weight %, more than 0.4% up to 1.0%, which is disclosed in Japanese Patent Disclosure No. 2000-319753.
The above described low carbon sulfur-free-cutting steel is a free-cutting steel of improved machinability in turning by containing soft MnS having a melting point around 1600xc2x0 C. in dispersed form in the matrix thereof, and the MnS inclusion is utilized as the lubricant to decrease friction between the edge of tool and the matrix. In the case where the machinability in turning is the only concern among various machinabilities, the problem can be solved by having much amount of MnS formed in the steel.
The MnS inclusion particles, however, are easily elongated during rolling or forging, and therefore, when the steel containing much amount of MnS is machined by turning, the elongated MnS comes out of the matrix of steel to adhere to the edge of tool and a built-up edge is formed, which tends to grow. If the built-up edge grows, it may adhere to the turned surface, and this process may be repeated. Thus, there is a problem that the turned surface may be roughened and deteriorated, and the machined product will be of poor surface condition.
In order to avoid deterioration of the turned surface it is necessary to slow down the turning speed at the finishing machining. This causes lowering the producing efficiency and results in increased manufacturing cost.
Thus, there has been demand for such a free-cutting steel that has good machinability in turning and gives small surface roughness after turning. The inventors, with the intention to meet this demand, conducted research and development, and invented a novel free-cutting steel with addition of a suitable amount of Ti to form carbosulfide-type inclusions by combining Ti with C and S. The invention was already disclosed (Japanese Patent Application 2001-167120). The steel consists essentially of, by weight %, C: 0.03-0.20%, Si: up to 0.2%, Mn: 0.5-3.0%, P: 0.02-0.40%, S: more than 0.2% up to 1.0%, Ti: 0.01-3.0%, Al: up to 0.005%, O: 0.0005-0.040%, Pb: less than 0.01% and the balance of Fe and inevitable impurities, and is characterized by Ti-based carbosulfide inclusions, typically, Ti4C2S2 therein.
Our further research revealed the facts that the contents of Si and Al are not so important in the free-cutting steel and that, if the O-content is at a higher level in the above range, macro-streak-flaw tends to occur. Occurrence of the macro-streak-flaw is an important problem from the viewpoint of steel quality and must be prevented. At such a decreased 0-level as 0.005% or less the macro-streak-flaw is no longer a problem, and it was ascertained by our research that the desired machinability is attainable at this low O-content. Our research further ascertained that Zr has the same effect as Ti, and therefore, a part or whole of Ti can be replaced by Zr.
The object of the invention is to provide, by utilizing the novel knowledge concerning the above-described free-cutting steel containing Ti-based carbosulfide inclusion, an improved free-cutting steel which has good machinability, particularly in turning, and small surface roughness after being turned, and no substantial problem on the macro-streak-flaw thereof.
The free-cutting steel according to the present invention, which achieves the above-mentioned object, contains, by weight %, C: 0.03-0.20%, Mn: 0.5-3.0%, P: 0.02-0.40%, S: more than 0.2% up to 1.0%, one or both of Ti and Zr (in case of both, the total amount): 0.01-3.0%, O: 0.0005-0.0050% and Pb: less than 0.01%, the balance being Fe and inevitable impurities, and the steel containing, Ti-based and/or Zr-based carbosulfide compound or compounds as the inclusion therein.